Trouble Shooter Just about the time you think you understand something, someone comes along to challenge your previously held assumptions. It can be confusing, but that’s usually a good thing. Hollow Log Karl Seyfert [email protected] I received several responses to my request for additional input at the end of my May column. To refresh your memory, the problem involved a 2000 Chevy Silverado 1500 pickup with about 185,000 miles on it. The Check Engine light was on and a DTC P0137 (bank 1 sensor 2 low voltage) was stored in memory. With the engine idling, bank 1 sensor 2 produced a steady 40mV, and sensor output voltage fluctuated slightly on acceleration. The bank 2 sensor 2 output voltage fluctuated normally. Swapping the harness connectors between bank 1 sensor 2 and bank 2 sensor 2 resulted in a bank 2 sensor 2 fault, and the PCM set another DTC after the second drive cycle. The exhaust gas readings at idle were .1% CO and 75 ppm HC. At cruise, the readings were .2% CO and 42 ppm HC. Manifold vacuum readings were 18 in. at idle and 19 in. at cruise. Next, the bank 1 sensor 2 harness connector was unplugged, then a new sensor was plugged into the harness without installing it in the exhaust. Reading ambient air only, the new sensor output was higher than when installed and the engine running. Soon afterward, it was discovered that the bank 1 converter was empty. After installing a new converter and oxygen sensor, bank 1 sensor 2 switched rich/lean/rich as it should and no further DTCs were stored. Several readers suggested that some mischief must have taken place before the truck was brought into the shop. After all, converters don’t normally shed their internals unless they’ve been severely overheated. And even then, some evidence of the converter monolith should have been found downstream; possibly bits and pieces of it should have been plugging the muffler. Tom’s customer, the owner, had only recently purchased the truck, and indicated that the Check Engine light was lit at that time, so he’s probably blameless. Perhaps the previous owner did the deed. Reader Steve Petry suggested that the sensor’s original low, fixed output at idle may have been caused by a damaged sensor heater. Without a heater, the sensor gets warm enough to operate only when there’s increased exhaust flow. This matches Tom’s description of the original sensor’s operation during acceleration. The heater may have been damaged at the same time the converter was overheated and damaged. But why was the converter on only one bank damaged? A severe bank 1 misfire? Tom indicated Photo: Karl Seyfert continued on page 8 Conventional oxygen sensors like this one do a pretty good job of indicating an engine’s air/fuel ratio. They’re most accurate when the engine is close to stoichiometry and less accurate at the rich and lean ends of the spectrum, which is why they’re being replaced. 6 July 2010 Trouble Shooter he had checked the bank 1 sensor 2 heater circuit; perhaps he missed something. The heater should have been resistance and current draw tested, then compared to the bank 2 sensor 2 heater. Reader Bob Russo wrote to say he’d recently worked on a Toyota truck that was storing a DTC P0420 (low cat efficiency). The precat oxygen sensor was cycling normally and the postcat sensor was showing a low, relatively constant signal of around 250mV. He tried everything, including an oxygen sensor replacement, but the code would always reappear. When he removed the And the Winner is... The PRO-Alert 2791 TM This award-winning electronic refrigerant leak detector utilizes a state-of-the-art infrared sensor for enhanced sensitivity and long life! 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The owner stated that he had been driving the truck for months before the code showed up. After Bob replaced the converter, all was fine. This example doesn’t help us explain Tom’s problem; it only indicates that similar things can happen when the converter is empty. Bob replaced the rear sensor before installing a new converter, so a heater problem can probably be ruled out. Why didn’t the rear sensor output track the front one? Bob chalked it up to another weird day in the shop. The last note I’ll share came from Fred Wilson, who asks us to think of oxygen sensors differently than we may be accustomed. In my previous column, I stated that low oxygen content = high oxygen sensor voltage reading = rich fuel mixture, and high oxygen content = low oxygen sensor voltage reading = lean fuel mixture. This is the way I’ve been taught to understand oxygen sensor function. And when a catalytic converter is functioning properly, I’d also been taught that most of the available oxygen is absorbed by the converter, leaving very little to be measured by the postcat sensor. Here’s Fred’s response: “These statements imply that the sensor measures exhaust gas oxygen content. We have been taught this concept for years. But it does not hold up to careful examination of how the sensor works. The sensor catalyzes excess hydrogen in the exhaust stream. It catalyzes the hydrogen using oxygen from the exhaust gases. If there is excess hydrogen and low oxygen content (a rich mixture), the sensor draws additional oxygen from the outside air. This outside oxygen flow is what the sensor is sensing. The more of this oxygen that is needed, the higher the sensor voltage that is produced, up to a limit of about 1 volt.” Thanks to all M OTOR readers for your responses. This discussion points up the limitations of the conventional oxygen sensor, and makes it easier to understand why a more responsive and accurate replacement was needed. For more on that, refer to Sam Bell’s article on air/fuel sensors (the oxygen sensor’s replacement), on page 28.